Strained silicon forming method with reduction of threading dislocation density

ABSTRACT

A method for growing strained Si layer and relaxed SiGe layer with multiple Ge quantum dots (QDs) on a substrate is disclosed. The method can reduce threading dislocation density, decrease surface roughness of the strained silicon and further shorten growth time for forming epitaxy layers than conventional method. The method includes steps of: providing a silicon substrate, forming a multiple Ge QDs layers; forming a layer of relaxed Si x Ge 1-x ; and forming a strained silicon layer in subsequence; wherein x is greater than 0 and less than 1.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a strained silicon substrate and theforming method thereof, more particularly, to a strained siliconsubstrate adapted for high-speed electronic and optical elements and theforming method thereof.

2. Description of Related Art

A strained silicon material is functioned as a substrate to varioushigh-speed electronic elements or optoelectronic elements due to itshigher electron mobility, i.e., higher carrier mobility. A strainedsilicon material is also applied as growing buffer layers for bondingIII–V based semiconductors and IV based semiconductors, so as tointegrate III–V elements (i.e. elements of group III or group V ofperiodic table) and IV elements (i.e. elements of group IV of periodictable) or to grow III–V elements on a silicon substrate. Since astrained silicon substrate can be integrated to grow III–V elements andIV elements to form semiconductor elements on a strainedsilicon-germanium epitaxy layer, and a strained silicon substrategenerally replaces a silicon substrate as a substrate for growinghigh-speed electronics, it is commonly referred to a virtual-substrate.

Typically, a virtual-substrate is generally formed by forming a strainedsilicon layer over a silicon-germanium epitaxy layer on a siliconepitaxy layer. FIG. 1 is a view of a structure of the virtual-substrate.As shown in FIG. 1, the structure includes a Si substrate, a Si buffer101, a graded silicon-germanium epitaxy layer 102, a silicon-germaniumepitaxy layer 103 and a strained silicon layer 104 sequentially.

Generally, conventional silicon-germanium epitaxy layers are grown on asilicon substrate through component graded growing. The stress betweensilicon-germanium epitaxy layer and silicon layer is reduced by such arelaxed mechanism of component graded silicon-germanium epitaxy layer.However, when better-relaxed effect is needed, the growing time ofsilicon-germanium growth is too long and the thickness of a relaxedsilicon-germanium epitaxy layer is too high. In addition, the alignmentof pattern formation through lithography is difficult, too. Hence, noadvantage in mass-production can be taken. Furthermore, asilicon-germanium epitaxy layer formed by such component-graded growthhas high surface roughness, threading dislocation density and defectivedensity, so that operation ability of electronic elements (oroptoelectronic elements) to be grown completely is relatively weakened.

As cited, a heterogeneous graded epitaxy layer of thicksilicon-germanium with low defective density grown by high temperatureis disclosed in U.S. Pat. No. 5,221,413, which does not improve priorhigh thickness and long growth time as well as mass-production. Inaddition, the cited high roughness is improved by selecting specialsubstrate materials (U.S. Pat. No. 6,033,803). However, mass-productionof high-speed electronic elements or optoelectronic elements byimproving high thickness, high roughness and long growth time at thesame time does not present in current processes or products.

Therefore, it is desirable to provide an improved method to mitigateand/or obviate the aforementioned problems.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a forming method ofstrained silicon substrate, which uses multiple quantum dots (QDs) todecrease defective density, reduce or relax strain and surface roughnessof epitaxy layers, and further shorten growth time for forming epitaxiallayers, thereby enhancing operation properties of device.

Another object of the present invention is to provide a multilayersubstrate with strained silicon, which can decrease defective density,reduce surface roughness of epitaxy layers and further shorten growthtime for forming epitaxy layers, thereby improving operation propertiesof device.

A further object of the present invention is to provide a stain relaxedmechanism of silicon-germanium epitaxy layers, which can decreasedefective density, reduce surface roughness of epitaxy layers, provide asubstrate for forming electronic elements and further improve operationproperties of high-speed electronics.

To achieve the object of the present invention, the forming method ofstrained silicon substrate includes: providing a silicon substrate;forming a silicon-germanium buffer layer with multiple Ge quantum dot onthe silicon substrate; forming a Si_(x)Ge_(1-x) layer on the bufferlayer, where 0<x<1; and forming a strained silicon layer on theSi_(x)Ge_(1-x) layer, where 0<x<1 .

The inventive strained silicon substrate includes: a silicon substrate;a strained silicon layer; a silicon-germanium buffer layer with multipleGe quantum dots sandwiched between the silicon substrate and the relaxedSi_(x)Ge_(1-x) layer strained silicon layer; and a Si_(x)Ge_(1-x) layersandwiched between the strained silicon layer and the silicon-germaniumbuffer layer with multiple quantum dots, where 0<x<1.

Other objects, advantages, and novel features of the invention willbecome more apparent from the following detailed description when takenin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a typical virtual-substrate ofgraded silicon-germanium epitaxy layers;

FIG. 2 is a cross-sectional view of a strained silicon substrate withlow dislocation according to an embodiment of the invention;

FIG. 3 is a cross-sectional view of the detail of FIG. 2 according tothe embodiment of the invention;

FIG. 4 is a cross-sectional view of strained silicon, with the aid of atransmission electron microscope, according to the embodiment of theinvention;

FIG. 5 is an enlarged cross-sectional view of strained silicon, with theaid of a transmission electron microscope, according to the embodimentof the invention;

FIG. 6 is a graph of a peak-Raman shift relation, produced by changingnumber of multiple germanium/silicon bilayers in strained siliconsubstrate, according to the embodiment of the invention;

FIG. 7 is a graph of X-ray diffraction spectrum of strained silicon andrelaxed SiGe according to the embodiment of the invention; and

FIG. 8 is a graph of an X-ray-peak to number-of-layer relation, producedby changing number of multiple germanium/silicon bilayers in strainedsilicon substrate, according to the embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Quantum dots adapted for the buffer layer with multiple Ge quantum dotsof the present invention can be any materials of III–V quantum dots(i.e. quantum dots of elements of group III or group V of periodictable) or IV quantum dots (i.e. quantum dots of elements of group IV ofperiodic table). Preferably, quantum dots adapted for the buffer layerwith multiple Ge quantum dots of the present invention are germanium(Ge) quantum dots, silicon-germanium quantum dots, silicon:carbonquantum dots or silicon-germanium-carbon quantum dots. Average spacebetween quantum dots in each layer is not limited in the buffer layerwith multiple silicon-germanium quantum dots, preferably in a range of10˜50 nm. Density of quantum dots is also not limited in the bufferlayer with multiple silicon-germanium quantum dots layers, preferably ina range of 10¹⁰˜10¹¹ cm⁻². Layer number of quantum dots is also notlimited in the buffer layer with multiple silicon-germanium quantumdots, preferably below 20 layers, even below 10 layers. Thickness ofsilicon spacer layer between germanium/silicon bilayers is also notlimited in the buffer layer with multiple silicon-germanium quantumdots, preferably in a range of 30˜50 nm. A method for forming quantumdots in the buffer layer with multiple silicon-germanium quantum dotscan be any appropriate method for growing epitaxy quantum dots in thisfield, preferably chemical vapor deposition (CVD), ultra-high vacuum CVD(UHV/CVD) or molecular epitaxy. Shape of a quantum dot is not limited inthe buffer layer with multiple silicon-germanium quantum dots,preferably in a semi-sphere or a drop-shape. Most threading dislocationdensity of silicon-germanium buffer layer is below 10⁹ cm⁻² in thebuffer layer with multiple silicon-germanium quantum dots, preferablybelow 5×10⁵ cm⁻². The inventive strained silicon can be applied toproduce various electronic elements or devices, preferably tooptoelectronic elements or high-speed electronic elements.

An embodiment for forming stained silicon substrate is given in thefollowing for better understanding.

[Formation of Strained Silicon Substrate of the Embodiment]

FIG. 2 is a structure of a strained silicon substrate with lowdislocation. As shown in FIG. 2, the structure includes steps of forminga silicon buffer layer 202 on a silicon substrate 201, forming a set of10 layers with Ge quantum dots 210 (FIG. 3) in the silicon buffer layer202, which is separated by a spacer forming of a 20-nm-thick Si layer203, and forming a relaxed silicon-germanium layer 204 and then astrained silicon layer 205 over the Ge quantum dots 210. The citedquantum dots are further shown in FIG. 3.

The inventive method for forming a strained silicon structure with lowdislocation density firstly applies UHV/CVD to silicon growth layer bylayer for smoothening the surface of a chip. Sequentially, 10 layerswith quantum dots isolated by a 20-nm-thick Si layer are formed and afollowing 500 nm relaxed silicon-germanium layer is grown.

When system strain is relaxed, nucleus of mismatch dislocation is formedpreferentially at local areas of quantum dots because stress iscollected in the local areas, and thus relaxed silicon-germanium layerepitaxy layers with high-level strained relaxation and low defectivedensity are left.

[Property Verification for Strained Si Substrate]

The cited strained Si substrate with multilayers and low dislocationdensity is subjected to an atomic force microscope (AFM) for the surfaceroughness measurement. The resulting surface roughness is 3 nm muchsmaller than that (6 nm) of SiGe epitaxy layer using the prior gradedgrowth. Accordingly, the surface roughness of the inventive substratewith low dislocation density is superior to that of the SiGe epitaxyformed by the prior graded growth.

To better understanding, an electron microscope is applied to observethe strained Si substrate with multilayers and low dislocation density.As a result shown in FIGS. 4 and 5, the strained Si substrate has athreading dislocation density of 2×10⁵cm⁻² much smaller than that (1×10⁹cm⁻²) of the SiGe epitaxy formed by the prior graded growth.

When a Raman spectrum is applied to observe the strained Si substratewith multiple germanium/silicon bilayers and low dislocation density,its peak changes with changing number of layer. As a result shown inFIG. 6, the relaxation effect is better as increasing the number of Sibuffer layers with multiple Ge quantum dots. When an X-ray diffractionis applied to observe the strained Si substrate with multilayers and lowdislocation density, its relaxation is improved as compared to the priorSiGe epitaxy with the same thickness. As shown in FIG. 8, improved levelof the relaxation is more obvious as increasing the number of multipleGe quantum dots.

As cited, the strained Si substrate formed by the inventive methodobviously has lower defective density than those of the prior gradedSiGe buffer epitaxy and the roughness of the inventive epitaxy isrelatively reduced, thereby enhancing operation properties of high-speedelectronic elements (or optoelectronic elements). Besides, as comparedto the prior graded SiGe epitaxy, the invention has better relaxationand less thickness for growth is required for forming the inventivestrained Si substrate and further growth time for forming the epitaxy isrelatively reduced. Therefore, the inventive strained substrate can beapplied to operation properties of high-speed electronic elements for agrowth substrate.

Although the present invention has been explained in relation to itspreferred embodiment, it is to be understood that many other possiblemodifications and variations can be made without departing from thespirit and scope of the invention as hereinafter claimed.

1. A strained silicon substrate, comprising: a silicon substrate; astrained silicon layer; a silicon-germanium buffer layer with multiplesilicon-germanium quantum dots and multiple silicon layers, sandwichedbetween the silicon substrate and uniform Si_(x)Ge_(1-x) layer; and auniform Si_(x)Ge_(1-x) layer, sandwiched between the strained siliconlayer and the silicon-germanium buffer layer with multiplesilicon-germanium quantum dots, wherein 0<x<1.
 2. The strained siliconsubstrate as claimed in claim 1, wherein the silicon-germanium bufferlayer has a density of silicon-germanium quantum dots ranged between10¹⁰ cm⁻² and 10¹¹ cm⁻² in each layer.
 3. The strained silicon substrateas claimed in claim 1, wherein a thickness of silicon space betweensilicon-germanium quantum dots in the silicon-germanium buffer layerranges between 10 nm and 50 nm.
 4. The strained silicon substrate asclaimed in claim 1, wherein a thickness of the silicon layer in thesilicon-germanium buffer layer ranges between 20 nm and 50 nm.
 5. Thestrained silicon substrate as claimed in claim 1, wherein theSi_(x)Ge_(1-x) layer has a thickness ranged between 200 nm and 1000 nm.6. The strained silicon substrate as claimed in claim 1, wherein thesilicon-germanium buffer layer has a threading dislocation density lessthan 5×10⁵ cm⁻².
 7. The strained silicon substrate as claimed in claim1, further comprising an optoelectronic or electronic element with thestrained silicon substrate.